1. Field of the Invention
The invention is in the field of tanks, principally for underground fluid storage, and is particularly directed to such tanks which are of double wall construction.
2. Description of the Prior Art
Regulations in many states, and of the U.S. federal government currently require double wall construction for underground fluid storage tanks because of environmental considerations. Such double wall tank construction constitutes, in effect, an inner tank supported within an outer tank. The inner tank defines the primary, inner chamber which provides primary containment for fluid being stored, while the space defined between the inner and outer tanks defines an outer, secondary chamber which provides for secondary containment of the fluid in the event a leak should develop through the wall of the inner tank, as for example from corrosion, a faulty weld, or seismic or other mechanical stressing. One or more monitors are conventionally located in communication with the outer, secondary chamber between the two tanks, and any leakage from the inner tank into this outer, secondary chamber is directed toward one or more of such monitors which then provide an alarm signal indicating the leakage.
There are several different grades and types of underground storage tanks generally considered to be of double wall construction currently in use in the United States, and these are almost all of cylindrical construction and are layed on their side underground, i.e., have their cylindrical axes disposed generally horizontally. A full double wall tank with two complete cylindrical tanks, one inside the other, is designated a "Type Two" double wall tank. This has double end walls and 360.degree. double cylindrical wall protection. Another type of tank commonly referred to as a double wall tank is a "Type One" tank, commonly known as a "wrap tank." Here, the primary tank is cylindrical, with an outer sheet provided which gives double wall protection for approximately 330.degree. around the lower part of the tank, leaving the top part of the tank with only the single wall protection of the primary tank. The Type One wrap tank is utilized with the consideration that the greatest potential for failure of the primary tank is in its lower part, with a relatively small potential for failure at its top. While regulations of some states still allow use of the Type One wrap tank as a double wall tank, other, more progressive states such as California require the Type Two full double wall tank.
All underground storage tanks require access pipe fittings which extend through the top of the tank from the outside into the primary containment chamber within the tank. Typically for the storage of fuels such as gasoline and diesel fuel there are at least five such fittings required, a fill fitting, a turbine fitting for fuel extraction, a fitting for gauging, a vent fitting, and a vapor recovery fitting. The Type One wrap tank is a metal tank, and for such tanks the pipe fittings are conveniently welded into the single wall at the top of the tank. For a Type Two full double wall metal tank, the fittings conventionally extend through both walls at the top of the tank, being welded to both the outer and inner tanks.
However, many tanks which are considered full double wall tanks are currently being produced that are nonmetallic, having a wound filament fiberglass/resin construction. This filament winding fabrication process does not permit fittings to be integrally incorporated in a double wall nonmetallic tank when the two walls are being fabricated, and if the fittings are to be disposed directly through the two walls of the nonmetallic double wall tank, then it is necessary to drill holes through the two walls and bond the fittings to the walls from both sides, which is costly, difficult and unreliable. Therefore, for nonmetallic double wall tanks, the almost universal current procedure is to provide manway openings, and to provide metal covers for such openings through which the pipe fittings are welded. Only three fittings can be accommodated in a single manway cover, so where more than three fittings are required, which is usually the case, two manways and associated metal covers with fittings are conventionally provided with nonmetallic tanks.
For this reason, manways are currently employed in all filament-wound double wall nonmetallic tanks. Manways are also required for double wall metal tanks where interior servicing will be necessary during use of the tanks, as for example where frequent cleaning is required as in jet fuel tanks, where special materials are to be held in the tank, such as distilled water, or certain chemicals, and it is necessary to get inside the tank after it is completed to apply lining material within the tank, or in some cases where bladders are installed internally within the tanks after the tanks have been constructed to enable positive expulsion of tank contents by pressurization between the tank walls and bladder.
Prior to the present invention, manway covers for such double wall metal tanks requiring manways, as well as the aforesaid metal manway covers with fittings for fiberglass tanks, have conventionally been of single wall construction. Thus, although the tank itself might otherwise comply with regulations calling for a Type Two full double wall tank, whether it be of metal or fiberglass, with prior art double wall tanks there still remained only single wall containment at the manway or manways. Should a leak occur in the seal of such a single manway cover, such leak might go undected for an extended period of time, since it could not possibly be detected by a monitor or monitors located between the inner and outer walls of the tank, and being underground it would not be observable from the surface unless it became so serious as to cause earth movement. Although current governmental regulations do not point directly to manway covers in calling for Type Two full double wall tanks, because of the danger of only single wall containment at manway covers, it is contemplated that future regulations will require complete double wall containment at all points or regions on tanks, even at the manways.